Excerpt from The Dynamo, Vol. 1 of 2: Its Theory, Design, and Manufacture
The interval since the last edition has added but little to the theory of the Dynamo, although in its practical design and art of manufacture many changes have been brought about by the necessity for the utmost economy in materials and labour to meet the stress of keen industrial competition. Whether for continuous-current dynamos or alternators, the drum armature with toothed core has so far displaced all other types that it has become superfluous to consider at length any other kind. In consequence the ring, discoidal-ring and disc armatures, after a preliminary description sufficient to place them in their due relation to the drum, disappear from the pages of the present edition. The drum too is only considered in its modern toothed form, with but little mention of the older smooth-surface form which, with the two-pole horse-shoe magnet, is now practically obsolete except in a few special cases.
On the continuous-current side the carbon collecting brush still holds the field, but much work, both experimental and theoretical, has been done towards the elucidation of its action. The treatment of commutation and of the sparking limitations of the dynamo has therefore been recast and expanded in the light of later researches, especially those of Prof. E. Arnold, to whom the acknowledgments of all students of this difficult problem are due. Within recent times the continued endeavour to reduce the necessary size and therefore cost of dynamos has led to a remarkable revival of the use of auxiliary commutating poles, with results that have amply justified their re-introduction. Their theory has therefore here been considered more fully and at such length as space permitted. The mechanical construction of the commutator, both of the V-type and with shrink-rings, has also been treated in greater detail in view of the higher speeds which now obtain.
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